Filters for use in air conditioning systems of the kind commonly provided in office, residential and healthcare buildings typically employ as media nonwoven fiber mats made for example from polyester, cellulosic or glass fibers and are designed to filter airborne particles from air flowing through the media. In use, as the particulate material is filtered from the air stream, the filtrate builds up on the filter surface and in pores of the matt, with the consequence that the filter progressively presents increasing resistance to air flow. Eventually, the air resistance becomes so great that the filter must be removed for cleaning or replacement.
Filter cleaning is costly and inconvenient both in terms of labor cost and system downtime. It can also be dangerous due to the possibility of infection from build up of bacterial and fungal growth on the filtrate. Replacement of the filter is generally a significant cost. It is therefore highly desirable to extend the time interval between successive filter replacements and/or maintenance service as long as possible, provided that the energy costs of circulating air through the filter can be maintained within specification.
Filters are generally designed in terms of parameters such as fiber type, fiber diameter, fiber matt thickness, matt density/porosity, construction and other physical and mechanical properties to provide a structure which is effective at filtration down to a specified particle size, but which at the same time will not unduly restrict airflow and which will clog slowly rather than quickly. These conflicting desiderata are met by compromise among the physical and mechanical parameters to provide a filter construction of a specific “efficiency”.
It is usual to “grade” filters under standardized test conditions which measure particle retention for particles of a given size (or size distribution) and given airflow The need to manufacture different grades to cater for specified requirements of different air-conditioning systems adds greatly to filter manufacturing and inventory costs. The filter grades applicable in Australia are G1, G2, G3, G4, F4, F5, F6, F7, F8, F9. This list is in order of increasing filter efficiency. Thus grade F9 is equivalent to a hepafilter (filters to below 0.2 microns), while G1 would filter out pigeons. These filter grades are more particularly defined in Australian Standard AS 1324.1
The present invention stems from the surprising discovery that the efficiency of a given filter construction can be improved by a chemical treatment so that for example a grade G3 filter prior to treatment is upgraded to perform at grade G4 or better level. The cost of the chemical treatment is significantly lower than the additional cost of manufacture of a higher grade filter. For example the cost of the chemical treatment is less than the additional cost of a grade G4 filter in comparison with a grade G3 in terms of its traditional construction. Furthermore, preferred embodiments of the invention allow a reduction in inventory since a number of “grades” of filter can be provided based on a single mechanical construction simply by varying the level and/or nature of chemical treatment applied to that structure.
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Accordingly it is an object of the invention to provide an improved filter and/or means of retro—improving the efficiency of an existing filter by relatively simple means.
It is an object of preferred embodiments of the invention to upgrade a given construction of filter by one grade or more by applying a simple chemical treatment.